This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Subjects who have participated in a prior gene transfer study will be asked to participate in this study. The purpose of this study is to complete follow-up and to learn about the long-term side effects of gene therapy. Participation in this study is expected to last for at least 15 years. The length of follow up may be extended in the future. Because there is so little experience with using gene transfer for treatment, all subjects who participated in gene transfer studies are expected to participate in long-term follow up. This is done so that any late problems that could be related to gene therapy can be evaluated. As part of this follow-up, investigators want to collect a blood sample each year that could be tested later for replication-competent retrovirus (RCR, virus from the original gene therapy that may be able to divide later and infect new cells on its own). These samples would be tested if there were a reason to suspect RCR. Gene therapy has been used for only a few years, and, because there is so little experience with this form of treatment, there is incomplete information about long-term safety. In one study which used a retrovirus gene transfer method, cancers occurred at approximately 3 years after treatment. This was associated with the retrovirus used. It is not known whether gene therapy can cause other late side effects.